Despite recent advances in the identification of the ionic composition of inner ear fluids and the localization within the cochlea of enzymes, e.g., Na-K-ATPase, known to be involved in ion and water transport in other organ systems, the regulation of inner ear fluids remains unresolved. Based upon evidence that changes in levels of certain hormones (e.g., adrenal steroids) can affect both the Na-K-ATPase activity and plasma membranes of renal transporting epithelia, it can be conjectured that fluid and ionic microhomeostasis of the cochlear duct may be affected similarly by adrenal steroids. The objective of this investigation is to examine the effect of adrenal steroids on Na-K-ATPase activity and cellular morphology of epithelial cells of the cochlear duct. Individual and combined concentrations of aldosterone and corticosterone will be given to adrenalectomized animals. Both quantitative and morphological techniques will be employ to determine the Na-K-ATPase activity in cochlear tissues of treated and control animals, and to examine the morphological integrity of cochlear epithelia by transmission electron microscopy. Na-K-ATPase activity will be determined by an established, highly sensitive fluorometric microassay originally developed for microdissected nephron segments. Surface membrane areas and cell volumes of select epithelial cells of the cochlear duct from experimental animals will be determined by a computerized image analysis system and compared to those of controls. Serum and urinary electrolyte balance and adrenal steroid levels will be determined and correlated. Such data will provide evidence as to the effect of hormones, e.g. adrenal steroids, on cochlear cells involved with endolymph maintenance. The hypothesis to be tested is that hormonal changes can affect the Na-K-ATPase activity in the epithelium of the cochlear duct, leading to a fluid and ionic imbalance which may contribute to cochlear dysfunction.